Hexamethylcyclotrisiloxane, otherwise known as cyclic trimer or D.sub.3 in the silicone art, can be anionically polymerized, and subsequently capped with a functional silane, to prepare a so-called "macromonomer" having a very narrow molecular weight distribution (see, for example, U.S. Pat. No. 5,399,649 to Okawa and European Patent Publication No. 0 664 492 A2 to Shin-Etsu Chemical Co.). These macromonomers, in turn, find utility in the synthesis of graft or block interpolymers with various organic systems. However, in order to be effective as a starting material, the hexamethylcyclotrisiloxane must separated from octamethylcyclotetrasiloxane and other impurities generally found in the crude feeds containing this product.
In commercial production, a crude mixture containing hexamethylcyclotrisiloxane may be recovered, e.g., as a byproduct from the hydrolysis of dimethyldichlorosilane, typically in the presence of an organic solvent. Although the primary object of this process is the formation of hydroxy-terminated polydimethylsiloxane and octamethylcyclotetrasiloxane, significant quantities of hexamethylcyclotrisiloxane and, to a lesser extent, cyclopolysiloxanes having greater degrees of polymerization, are also formed under the highly acidic hydrolysis conditions. The crude mixture of volatiles obtained from the hydrolysis process must then be distilled to separate the hexamethylcyclotrisiloxane from the octamethylcyclotetrasiloxane, higher cyclopolysiloxanes and other impurities. Likewise, when the crude blend is obtained from a base-catalyzed equilibration of a polydimethylsiloxane or mixture of polydimethylsiloxanes, it must also be distilled to separate the hexamethylcyclotrisiloxane from the higher cyclopolysiloxanes and other impurities.
When such an operation is attempted using conventional fractional distillation equipment comprising a reboiler, column and condenser, solid hexamethylcyclotrisiloxane (melting point=64.degree. C.) starts to deposit in the condenser and eventually blocks or severely restricts overhead throughput. Of course, changes in condenser design can be implemented to overcome this difficulty. Likewise, the distillation can be carried out at an elevated pressure (and condenser temperature) to minimize this effect. But both of these remedies require additional capital outlay and are not generally compatible with existing equipment constraints.
There is therefore a need for a method which will allow the isolation of hexamethylcyclotrisiloxane which is suitable for use in the synthesis of macromonomers from the above described crude blend using conventional distillation equipment.